Previously, the development of Asterias embryos in a microdrop preparation, as described by Lutz and Inoue, has displayed patterns which may be explained by limitation of oxygen transport through the experimental system. Specifically embryo development, as indicated by the rate of cell division, decreased as a function of the distance of the embryo from the air/water interface at the edge of the microdrop. Understanding this phenomenon is important in our understanding of developing embryos. The goals of this experiment were to demonstrate a relationship between the patterns of embryo development in the preparation and the density of oxygen consuming cells. Oxygen consumption was also monitored in individual embryos using the self-referencing oxygen microelectrode system. The results of these experiments demonstrated that the patterns of development observed in the preparation were dependent upon the density of the embryos. A high density of cells resulted in the production of a very shallow zone of normally developing embryos near the air/water interface at the edge of the microdrop, with the majority of cells showing complete inhibition of growth. At lower concentrations the zone of normally developing embryos was larger with only the center most embryos showing signs of complete inhibition of growth. By monitoring the rate of oxygen consumption in individual embryos we were able to determine if differences in oxygen consumption at different stages of development might be contributing to the development of these patterns. This data showed that when the oocyte is fertilized the rate of metabolic oxygen consumption increased to a new level approximately 2.1 times higher than that measured in the unfertilized oocyte. This level of metabolism was constant through the subsequent stages of embryonic development monitored during these experiments. Taken together the results show that inhibition of normal growth and development within a microdrop preparation may occur as a result of limitations in the system's ability to deliver oxygen to rapidly respiring cells. For Arbacia punctulata the inhibition of normal development within the system was not influenced by changes in metabolism during development. This study also documents, for the first time, the activation of metabolic activity during fertilization of Arbacia punctulata oocytes as measured in individual embryos.